Low northern satellite (4)
I believe the answer is:
moon
'satellite' is the definition.
(moon is a kind of satellite)
'low northern' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'moo'+'n'='MOON'
(Other definitions for moon that I've seen before include "Reveal rear" , "Space traveller" , "low point" , "Orbiting body" , "drummer?" .)